1. Field of the Invention
The invention relates to an ion guide chamber, comprising an elongate chamber, at least one first electrode for generating a field for transporting ions along said elongate chamber and at least one second electrode for generating a field for focusing the ions within the elongate chamber. The invention further relates to an apparatus for mass analysis comprising such an ion chamber.
2. Description of the Related Art
Mass spectrometry (MS) is a method of analysis that can be applied in a wide field of different applications. MS can be used for chemical and biological analysis in many different fields, including the analysis of gases, liquids, solids, plasmas, aerosols, biological aerosols, biological material, tissue, and so forth.
Mass spectrometry involves the measurement of the mass-to-charge ratio of ions. In many applications these ions are created in high pressure ion sources. Many mass analyzing devices however require that the ions are injected into a high vacuum chamber. Therefore, it has been proposed to transfer the ions from the high pressure ion source into the high vacuum through an intermediate pressure region. Often, the ions have to pass one or several differentially pumped stages for the transfer into the high vacuum of the MS.
It is desirable that this transfer of ions is efficient, e.g. with little loss of ions. Various methods have been used to optimize the transmission. Since the differential pumping stages often consist of one or several orifices or capillaries through which the ions have to be transferred, many of the inventions for increasing ion transmission incorporate ways to retain the ions close to the ideal ion path connecting those orifices and capillaries.
This is often accomplished with an ion guide chamber that holds two superimposed fields. A first field is used for transport of ions through the residual gas from the entrance to the exit. For this, the field direction is essentially parallel to the chamber main axis, and the field can be static. A second electric field is applied for confining the ions close to the axis. This is often done with an RF field with low amplitudes on the chamber axis and larger amplitudes away from the axis. Such an RF field creates an effective potential confining the ions to the axis. Examples of such fields are RF multipole fields. The transport field controls the axial ion movement and directs the ions towards the exit orifice into the (next) higher vacuum, whereas the RF field confines the ions to the center axis within the chamber.
An example of such a device is described in U.S. Pat. No. 4,963,736 (MDS Inc.) as well as in Douglas J. D. and French J. B., Collisional Cooling effects in radio frequency quadrupoles, J. Am. Soc. Mass Spectrom. 3, 398, 1992. It uses radio frequency (RF) fields, which can focus the ions along an axis and additionally can cool the ions through collisions to further increase transmission efficiencies into the mass spectrometer. The fields are generated by elongated rods that are arranged within the vacuum chambers.
Another device is described in U.S. Pat. No. 5,847,386 (MDS Inc.) and in Dodonov A., Kozlovsky V., Loboda A. Raznikov V., Sulimenkov I., Tolmachev A., Kraft A., Wollnik H., A new Technique for Decomposition of Selected Ions in Molecule Ion Reactor Coupled with Ortho-Time-of-flight Mass spectrometry, Rap. Comm. In Mass Spec., 11, 1649-1656, 1997. This device also uses an RF quadrupole but also has a superimposed linear field along the RF Quadrupole by segmenting the quadrupole. This allows to control ion energies and to decrease the residence time in the quadrupole. Again, the quadrupole rod sets are arranged within the vacuum chamber.
In still other devices the superposition of a linear field and an RF field is achieved by tilting the quadrupole electrodes towards the central axis, or by using quadrupole electrodes of tempered shape instead of cylindrical shape.
The geometry of the prior art vacuum chambers and rods is rather complex. Furthermore, one has to make sure that contamination of the RF electrodes to the analyte gas held in the vacuum chambers, e. g. due to outgassing, does not occur. This sets high demands on the RF electrode material. Furthermore, breakdown voltages are very low at intermediate pressures as they are used within the vacuum chambers described above. Therefore, discharges may be provoked by the RF electrodes arranged within the chambers.